A high-speed, high-resolution laser printer which is used, for example, for medical image printing, may employ a resonant scanning system. The scanning system directs a laser beam horizontally along each "row" of a stationary print medium, for example, a photographic film, with the beam being selectively pulsed, or turned on, and off, in order to direct light at particular points, or pixels in the row. The pixels receiving the light are recorded as such on the print medium.
The resonant scanners directs the laser beam across each row of pixels using an oscillating mirror. As the mirror oscillates in one direction it directs the beam to successive horizontal, or x-axis, locations, effectively directing the beam to scan across the film. The laser is not operated as the mirror oscillates in the opposite direction, and thus, printing occurs in one direction only. The mirror repeats its oscillations for each row of pixels.
The mirror oscillates in a sinusoidal manner, that is, it begins its oscillation at one side of the film or horizontal axis, speeds up as it directs the beam along the axis toward and past the middle of the film and slows down as it directs the beam to the far side of the film.
In order to print images which are not distorted by the sinusoidal movement of the mirror, as discussed in Mecklenburg, Medical Imaging, Proceedings of the Society of Photo-Optical Instrumentation Engineers (1987), Volume 767 pgs. 536-542, a "non-linear" clock is typically used to time the laser pulses. Specifically, the non-linear clock operates in synchronism with the oscillating mirror such that the laser is pulsed at times which correspond to uniform distance intervals in the row being scanned. The clock rate must thus be proportional to the angular velocity of the mirror. Such a clock is discussed in U.S. Pat. No. 4,541,061 to Schoon.
The Schoon system uses a voltage-controlled oscillator ("VCO") to clock the laser pulses. Basically, the VCO output signal acts as a clock for a pixel counter which generates a count corresponding to the next pixel location with which the laser beam will be aligned. The counter addresses (i) a first memory which contains information relating to the desired image, and (ii) a second memory which contains information relating to the angular velocity of the mirror at the time. The output of the first memory controls the pulsing of the laser, such that it is pulsed each time a dot is desired at the pixel location with which the beam is then aligned. The output of the second memory controls, through a digital-to-analog converter ("DAC"), the frequency-control voltage applied to the VCO. The second memory thus controls the rate at which the counter counts.
The operations of the VCO and the pixel counter are controlled by a scan signal which is asserted and de-asserted at particular points in a "scan cycle." A scan cycle begins with a "write" portion, which is a scan in the direction in which the laser printer writes on the film, and ends with a "return" scan which is a scan across the film in the opposite direction. Specifically, the scan signal is asserted at the start of the write portion of a scan cycle, and it enables operation of the VCO and the pixel counter. The VCO and the counter then operate together to produce clock and laser control signals. When the write portion of the scan cycle is over, the scan signal is de-asserted. This resets the pixel counter and inhibits the VCO, which prevents them from producing the clock and control signals. At the start of the write portion of the next scan cycle, the scan signal is asserted and the VCO and the pixel counter are again enabled.
It is imperative that the write portion of a scan operation begin at the same horizontal position for each line, and that it pulse the laser at the appropriate times to produce dots in the proper horizontal locations. Otherwise, the printed images will be distorted. Thus the operations of the clock and laser control mechanism must be precisely synchronized to the operations of the scanner.
The known prior art system turns the VCO, and thus the clock and laser controller, on and off at the beginning and end of the write portion of a scan cycle, respectively. Accordingly, when the write portion starts the VCO must be re-synchronized. The system may re-start the VCO when a detector detects the start of the write portion of the scan cycle. The VCO then synchronizes, over a period of time, to the frequency of the scanner. Alternatively, the system may include a separate VCO re-synchronization clock whose frequency is many times faster than the resonant frequency of the scanner. When the VCO turns on at the start of the write portion of the scan cycle, it synchronizes to the frequency of the re-synchronization clock. The VCO output signal is then fed to a divider circuit which generates the smaller clock for the pixel counter. Such a re-synchronization clock is both costly and difficult to build because it must run many times faster than the fastest clock signal required to drive the pixel counter. Further, the re-synchronization clock must continuously stay in synchronism with the scanner. Regardless of which synchronization mechanism the system uses, the system still requires synchronization of the VCO at the start of the write portion of each scan cycle. Accordingly, the system may produce distorted images while the VCO is synchronizing.
Distortion may occur, also, if the VCO produces an output frequency which is not linearly related to the applied frequency-control voltage. This may occur, for example, if the VCO is subjected to changes in ambient temperature. This non-linearity adversely affects the timing of the clock and control signals applied to the rest of the system.